Polypropylene homopolymers have been commercially produced since the early 1960s and have found wide application in molded articles, especially where a higher degree of heat resistance is required than is achievable by high density polyethylene compositions. Polypropylene homopolymers, however, suffer one significant drawback in that they are brittle at temperatures below approximately 10.degree. C. To overcome this problem, elastomeric components are typically blended or polymerized in situ, in a separate reactor, with the polypropylene homopolymer (or a propylene copolymer) to form what is referred to as an "impact polypropylene copolymer". In the in situ polymerization, this amounts to having an ethylene/propylene copolymer incorporated into a matrix of propylene homopolymer or copolymer. These impact polypropylene copolymers exhibit improved levels of low temperature impact strength. By choosing the correct compositional parameters, formulations with high impact strengths at low temperatures, having minimal loss in other properties, such as stiffness, can be achieved. Nevertheless, these formulations exhibit one very undesirable property known in the art as "stress whitening". This phenomena occurs when an impact polypropylene copolymer is impacted or stressed, and results in a whitened area distinctly noticeable in a molded article. Obviously, these marks are not acceptable in consumer items such as housewares, automobile interiors, and appliances. Because of the stress whitening problem, a technology has been developed to overcome the difficulty, and improvements have been achieved. These improvements, however, are realized at some sacrifice in impact strength and/or stiffness, or require the use of costly multiple (more than two) reactor systems.
For example, excellent stress whitening resistance can be accomplished in a three reactor process using various combinations of ethylene and propylene, but the third reactor increases production costs to prohibitive levels.
Excellent stress whitening resistance is considered to be measurably below 0.1 inch when an injection molded disc is impacted at ten inch-pounds, and the resulting white mark is measured after 24 hours of aging.
U.S. Pat. No. 4,882,380 teaches a process, which also results in a product having excellent stress whitening resistance and, in addition, higher stiffness. The low temperature impact strength, while satisfactory, particularly with regard to Gardner impact strength (the falling weight test - ASTM D-3029, Condition G), is somewhat deficient, however, insofar as "notch sensitivity" or Izod impact strength (also referred to as "notched Izod"), as measured by ASTM D-256, is concerned. In certain applications such as molded articles having sharp corners or notched or textured surfaces, it is desirable, however, to provide an even higher Izod impact strength even if some stiffness is sacrificed.